generated the required .ppk file and loaded it in Connection=>SSH=>Auth=>Private key file for authentication.
The connection ERROR message: Network Error: Connection refused.
Am i doing something wrong ?
The error message means that PuTTY can't even get as far as connecting to the ssh server and presenting the key. Things worth checking are if you can ping the host you're trying to connect to, and it's behind a router that the required port (usually 22) is forwarded, and that the ssh server is actually running. Or it could be you've been locked out due to too many failed attempts as Ryan said, in which case try from a different computer.
Are you on the same network as the server or are there routers in between?
Sounds like the SSH server is explicitly not allowing connections from you. Perhaps you tried too many times and got locked out?